424 research outputs found
A Spitzer View of the Young Open Cluster NGC 2264
We have performed mid-IR photometry of the young open cluster NGC 2264 using
the images obtained with the Spitzer Space Telescope IRAC and MIPS instruments
and present a normalized classification scheme of young stellar objects in
various color-color diagrams to make full use of the information from
multicolor photometry. These results are compared with the classification
scheme based on the slope of the spectral energy distribution (SED).
From the spatial distributions of Class I and II stars, we have identified
two subclusterings of Class I objects in the CONE region of Sung et al. The
disked stars in the other star forming region S MON are mostly Class II
objects. These three regions show a distinct difference in the fractional
distribution of SED slopes as well as the mean value of SED slopes. The
fraction of stars with primordial disks is nearly flat between log m = 0.2 --
-0.5, and that of transition disks is very high for solar mass stars. In
addition, we have derived a somewhat higher value of the primordial disk
fraction for NGC 2264 members located below the main pre-main sequence locus
(so-called BMS stars). This result supports the idea that BMS stars are young
stars with nearly edge-on disks. We have also found that the fraction of
primordial disks is very low near the most massive star S Mon and increases
with distance from S Mon.Comment: 38 pages, 21 figures, 5 tables (AJ accepted
Open clusters as laboratories: The angular momentum evolution of young stars
This is the annual status report for the third year of our LTSA grant 'Open Clusters as Laboratories.' Because we have now had a few years to work on the project, we have started to produce and publish a large number of papers. We have been extremely successful in obtaining ROSAT observations of open clusters. With the demise of the PSPC on ROSAT, our main data source has come to an end and we will be able to concentrate on analyzing those data
A Deep Imaging Survey of NGC 6475 and a Deep Survey of the Young Open Cluster Alpha Persei
We have published three papers directly related to the x-ray data (see below), and one other paper directly related to the optical photometry obtained in connection to the ROSAT data analysis (also listed below). In addition, we have used the x-ray data in combination with the optical photometry and with high resolution optical spectra that we have obtained (to derive spectroscopic rotational velocities) to address questions related to the dependence of coronal activity on the rotation rate, mass, and other characteristics of the target stars. The overall goal of our ROSAT projects has been to determine the time evolution of coronal activity in low mass stars, and from that behavior place constraints on the physical processes involved in generating coronal activity. NGC 6475 and Alpha Persei are important parts of that process. We are now beginning to address how these data constrain theoretical models of angular momentum evolution and dynamo magnetic field generation (see paper by Stauffer et al. and Krishnamurthi et al. listed below in the "related papers" section). We are still working on two papers pertaining to this grant (rotational velocities and chromospheric activity of Alpha Persei x-ray sources and rotational velocities and lithium abundances of NGC 6475 sources), but we will publish those papers using funds from other grants
The Formation and Evolution of Planetary Systems: Description of the Spitzer Legacy Science Database
We present the science database produced by the Formation and Evolution of
Planetary Systems (FEPS) Spitzer Legacy program. Data reduction and validation
procedures for the IRAC, MIPS, and IRS instruments are described in detail. We
also derive stellar properties for the FEPS sample from available broad-band
photometry and spectral types, and present an algorithm to normalize Kurucz
synthetic spectra to optical and near-infrared photometry. The final FEPS data
products include IRAC and MIPS photometry for each star in the FEPS sample and
calibrated IRS spectra.Comment: 64 pages, 12 figures, 5 tables; accepted for publication in ApJ
Government-Industry Cooperative Fisheries Research in the North Pacific under the MSFCMA
The National Marine Fisheries Service’s Alaska Fisheries Science Center (AFSC) has a long and successful history of conducting research in cooperation with the fishing industry. Many of the AFSC’s annual resource assessment surveys are carried out aboard chartered commercial vessels and the skill and experience of captains and crew are integral to the success of this work. Fishing companies have been contracted to provide vessels and expertise for many different types of research, including testing and evaluation of survey and commercial fishing gear and development of improved methods for estimating commercial catch quantity and composition. AFSC scientists have also participated in a number of industry-initiated research projects including development of selective fishing gears for bycatch reduction and evaluating and improving observer catch composition sampling. In this paper, we describe the legal and regulatory provisions for these types of cooperative work and present examples to illustrate the process and identify the requirements for successful cooperative research
Lithium in the Upper Centaurus Lupus and Lower Centaurus Crux Subgroups of Scorpius-Centaurus
We utilize spectroscopically derived model atmosphere parameters and the
\ion{Li}{1} subordinate line and the doublet to
derive lithium abundances for 12 members of the Upper-Centaurus Lupus (UCL) and
Lower-Centaurus Crux (LCC) subgroups of the Scorpius Centaurus OB Association.
The results indicate any intrinsic Li scatter in our 0.9-1.4 stars
is limited to dex, consistent with the lack of dispersion in
stars in the 100 Myr Pleiades and 30-50 Myr IC 2391 and
2602 clusters. Both ab initio uncertainty estimates and the derived abundances
themselves indicate that the 6104 line yields abundances with
equivalent or less scatter than is found from the 6708 doublet as a
result of lower uncertainties for the subordinate feature, a result of low
sensitivity to broadening in the subordinate feature. Because NLTE corrections
are less susceptible to changes in surface gravity and/or metallicity for the
6104 {\AA} line, the subordinate Li feature is preferred for deriving lithium
abundances in young Li-rich stellar association stars with K.Comment: Accepted for publication in Astronomical Journal (abstract shortened
for astro-ph submission
Characterizing the Variability of Stars with Early-release Kepler Data
We present a variability analysis of the early-release first quarter of data publicly released by the Kepler project. Using the stellar parameters from the Kepler Input Catalog, we have separated the sample into 129,000 dwarfs and 17,000 giants and further sub-divided the luminosity classes into temperature bins corresponding approximately to the spectral classes A, F, G, K, and M. Utilizing the inherent sampling and time baseline of the public data set (30 minute sampling and 33.5 day baseline), we have explored the variability of the stellar sample. The overall variability rate of the dwarfs is 25% for the entire sample, but can reach 100% for the brightest groups of stars in the sample. G dwarfs are found to be the most stable with a dispersion floor of σ ~ 0.04 mmag. At the precision of Kepler, >95% of the giant stars are variable with a noise floor of ~0.1 mmag, 0.3 mmag, and 10 mmag for the G giants, K giants, and M giants, respectively. The photometric dispersion of the giants is consistent with acoustic variations of the photosphere; the photometrically derived predicted radial velocity distribution for the K giants is in agreement with the measured radial velocity distribution. We have also briefly explored the variability fraction as a function of data set baseline (1-33 days), at the native 30 minute sampling of the public Kepler data. To within the limitations of the data, we find that the overall variability fractions increase as the data set baseline is increased from 1 day to 33 days, in particular for the most variable stars. The lower mass M dwarf, K dwarf, and G dwarf stars increase their variability more significantly than the higher mass F dwarf and A dwarf stars as the time baseline is increased, indicating that the variability of the lower mass stars is mostly characterized by timescales of weeks while the variability of the higher mass stars is mostly characterized by timescales of days. A study of the distribution of the variability as a function of galactic latitude suggests that sources closer to the galactic plane are more variable. This may be the result of sampling differing populations (i.e., ages) as a function of latitude or may be the result of higher background contamination that is inflating the variability fractions at lower latitudes. A comparison of the M dwarf statistics to the variability of 29 known bright M dwarfs indicates that the M dwarfs are primarily variable on timescales of weeks or longer presumably dominated by spots and binarity. On shorter timescales of hours, which are relevant for planetary transit detection, the stars are significantly less variable, with ~80% having 12 hr dispersions of 0.5 mmag or less
New BVI_C Photometry of Low-mass Pleiades Stars: Exploring the Effects of Rotation on Broadband Colors
We present new BVI_C photometry for 350 Pleiades proper motion members with 9 < V ≾ 17. Importantly, our new catalog includes a large number of K- and early M-type stars, roughly doubling the number of low-mass stars with well-calibrated Johnson/Cousins photometry in this benchmark cluster. We combine our new photometry with existing photometry from the literature to define a purely empirical isochrone at Pleiades age (≈100 Myr) extending from V = 9 to 17. We use the empirical isochrone to identify 48 new probable binaries and 14 likely nonmembers. The photometrically identified single stars are compared against their expected positions in the color-magnitude diagram (CMD). At 100 Myr, the mid K and early M stars are predicted to lie above the zero-age main sequence (ZAMS) having not yet reached the ZAMS. We find in the B – V versus V CMD that mid K and early M dwarfs are instead displaced below (or blueward of) the ZAMS. Using the stars' previously reported rotation periods, we find a highly statistically significant correlation between rotation period and CMD displacement, in the sense that the more rapidly rotating stars have the largest displacements in the B – V CMD
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